1. Field of the Invention
The invention relates to imaging of materials in living tissue, and in particular to increasing the power output of transducing means used for acoustic imaging by compensating for thermal problems associated with such increased power output.
2. Description of the Related Art
Medical ultrasound imaging has become a popular means for visualizing and medically diagnosing the condition and health of interior regions of the human body. With this technique an acoustic transducer probe, which is attached to an ultrasound system console via an interconnection cable, is held against the patient's tissue by the sonographer whereupon it emits and receives focused ultrasound waves in a scanning fashion. The scanned ultrasound waves, or ultrasound beams, allow the systematic creation of image slices of the patients internal tissues for display on the ultrasound console. The technique is quick, painless, fairly inexpensive and safe, even for such uses as fetal imaging.
In order to get the best performance from an ultrasound system and its associated transducers it is desirable that the transducers used to emit and receive ultrasonic pulses be capable of operating at the maximum acoustic intensity allowable by the U.S. Food and Drug Administration (FDA). This will help maximize the signal to noise ratio for the given system and transducer, help achieve the best possible acoustic penetration, and ensure that imaging performance is not limited by the inability to emit the full allowable acoustic intensity. At the same time, there are practical and regulatory limits on the allowable surface temperature that the transducer may attain as it performs its imaging functions. The Underwriters Laboratory (U.L.) Standard #UL544 "Standard for Safety: Medical and Dental Equipment" specifies an upper limit of 41.degree. C. for the transducer portion contacting the patient's skin. In addition, sonographers prefer to grip a transducer case which is comfortably cool, thereby preventing excess perspiration in their hands and a potential to lose their grip on the device.
Given that it is desirable to be able to operate at the maximum allowable acoustic intensity and also desirable to control the surface temperature distribution of the patient and user-contacting portions of the transducer's surfaces, thermal engineering is a serious consideration during transducer design. There are essentially two possible paths to proceed on with regard to transducer thermal engineering.
The first path makes use of passive cooling mechanisms and involves insuring that the heat that is generated both by the electroacoustic energy conversion process taking place in the transducer's piezoelements and by the acoustic energy passing through and/or into adjacent transducer materials is passively spread out to as large an external transducer surface area as possible. This heat spreading process is typically achieved internal to the transducer by thermal conduction through solid materials and subsequently from the transducer's external case employing natural free convection to the atmosphere. Ideally the external heat-convecting surface area would consist of the entire transducer's external surface area from which free convection cooling to the atmosphere can potentially take place in an unobstructed manner. Transducer manufacturers have thus incorporated various passively conducting heat-spreading plates and members inside the transducer's interior spaces to ensure the spreading of the heat to the entire transducer case surface. Such members work well, however, it is frequently the ability to get the heat out of the electroacoustic elements themselves and into such adjacent internal thermal-sinking structures such as these commonly used spreading plates that provides a significant portion of the probes total thermal dissipation resistance. If this internal thermal path is not a good one it is difficult to spread the heat generated by the piezoelements around the case. If the heat generated by the piezoelements cannot be removed, and effectively coupled and sunk to the entire transducer case area, then the probe surface portion in contact with the patient runs hotter than desired as this probe portion is directly adjacent the piezoelements. Thus, even in the passive strategy, there is concern concerning three key mechanisms: a) removing the heat from the highly localized piezoelement region; b) spreading said heat efficiently to the external case surfaces; and c) allowing for unobstructed natural convection from the warm transducer surfaces.
In any event, using this passive strategy, maximizing the external probe surface area onto which heat spreads in a fairly uniform manner minimizes the peak surface temperature attained anywhere on the probes surface during steady state convection of the probes heat to the ambient. This passive strategy amounts to spreading the heat load around to minimize the impact of the limited ability of free convection to dissipate heat. Its fundamental limitation is that, for most transducers, even if heat is spread uniformly on the external case surfaces, it only takes a few watts of transducer driving power to cause the average transducer surface temperature to become unacceptable either with respect to the patient or the sonographer. In these cases, and particularly for small transducers having small surface areas, one may find that one is unable to operate at the allowable acoustic intensity limit because of excessive temperatures.
FIG. 1 shows a prior-art medical ultrasound transducer 1 in schematic sectional view. Transducer 1 has a typically polymeric external case 2 which is gripped by the sonographer. The top of the transducer (+Y end) can be seen to have the typical acoustic lens 3 which serves to focus the ultrasound beam in the X-Y plane as it passes into the subject patient. Focusing in the Y-Z plane is done via electronic phase delays between the various piezoelements which are arranged on a Z-axis pitch and spacing passing into and out of the paper as is usual for phased array transducers. The bottom or back of the transducer 1 has emanating from it a flexible coaxial cable bundle 4. The cable 4 is shown in broken view at its midpoint to indicate its considerable length, usually on the order of 6 to 12 feet. Where cable 4 exits from the transducer 1, and specifically where it exits from the transducer case 2, can be seen a flexible strain relief 5. Strain reliefs are usually fabricated from a flexible rubber, such as silicone rubber, and they serve to prevent damage to the cable 4 or chemical leakage into the case 2 at the point of cable/case juncture particularly as cable 4 is flexed by the user.
A transducer cable connector 6 can be seen at the termination of the cable 4 (-Y end). The connector 6 is usually of a mass-actuated design and has an appropriate rotatable actuation knob 8 for that function. To the right of the transducer's connector 6 are shown in phantom a mating ultrasound system connector 7 mounted on an ultrasound system console 9. To use the transducer the sonographer would plug connector 6 into mating connector 7 (connectors shown unmated) thereby electrically connecting the transducer 1 to the ultrasound system console 9.
In the interior portion of the bottom of transducer 1, inside of polymeric case 2, portions of numerous electrical interconnects 10 (indicated by partial dotted lines) run from the transducer device 1 into the cable 4 and, in turn, into the connector 6. Generally a large number of interconnects 10 comprising coaxial wires of controlled impedance are provided in cable 4 to carry the electrical impulses transmitted to and received from the individual piezoelements making up the phased array. The details of how the interconnects 10 are mated to the piezoelements or to the connector are not shown as it is not critical to the understanding of this invention. It should be generally understood that numerous interconnects 10 pass from the transducer 1 and its piezoelements through the cable to the connector 6 and these serve an electrical function. Interconnects 10 must physically be routed through the interior of the back of the transducer case 2, and around whatever other means, thermal or otherwise, are located therein.
The electroacoustic transducer device assembly 50 is packaged and operated inside the confines of the polymeric case 2. Assembly 50 is shown schematically in FIG. 1 and in FIG. 7a. Assembly 50 comprises acoustic backer material 11, a piezoelements 12 and one or more (one shown) acoustic matching layers 13. Acoustic backer material 11 serves the functions of attenuating acoustic energy which is directed backwards to minimize reverberations and ringiness, and as a mechanical support for piezoelements 12. Materials used to fabricate backer 11 are generally poorly or only modestly thermally conductive as it is exceedingly difficult to design a highly thermally conductive yet acoustically highly lossy material. Piezoelements 12 may, for example, be fabricated from lead zirconate titanate (PZT) or composite PZT in a manner well-known to one of average skill. On top of piezoelements 12 is the matching layer or layers 13 which serve to act as an acoustic impedance transformer between the high acoustic impedance piezoelements 12 and the low acoustic impedance, human patient. (The human patient is not shown, but it should be understood that the patient is in contact with lens 3.)
The piezoelement material, typically PZT, is a ceramic having generally poor to modest thermal conductivity. The matching layer(s) 13 materials also frequently have poor to modest thermal conductivity because of their conflicting acoustic requirements. It is to be noted that the backer 11, the piezoelements 12 and the matching layer(s) 13 are all intimately bonded to each other and to the lens material 3 such that acoustic energy produced in piezoelements 12 may pass through the layer interfaces in the +Y-direction freely. Of course reflected acoustic echoes from the body may also likewise pass freely in the -Y direction, back into probe 1.
Not shown in FIG. 1 are horizontally running (+-X axis direction) electrodes in any of the interfaces of the type between lens 3 and layer 13, layer 13 and piezoelements 12 or piezoelements 12 and backer 11. Adequate thin electrodes must be present to apply and sense electrical potentials across the top and bottom surfaces of the piezoelements 12. Electrical interconnects 10 are typically routed and connected to such dedicated interface electrodes on a piezoelement by piezoelement basis (connections and routing not shown). The interface or surface electrodes are required to make electrical contact to each piezoelements 12 without appreciably negatively impacting the acoustic performance spectrum of transducer 1. Thus, such electrodes are typically chosen to be very thin, metallic, and have very little mass. This, in turn, causes the electrodes to be poor thermal conductors in the lateral X-direction.
Also shown in FIG. 1 are two symmetrically situated pairs of passive thermal conduction enhancement members 14 and 15 arranged on each side of assembly 50. Thermal member 14 is schematically shown physically and thermally connected to the edge region of element array 12 and layer 13, and possibly also to the ends of the interfacial or surface electrodes (not shown). Thermal member 15 is schematically shown thermally and physically connected to member 14. The members 14 and 15 are arranged to be in close juxtaposition and in good thermal contact with the interior walls of case 2. It will be noted that thermal member 15 may typically be thicker (as shown) and therefor more thermally conductive than member 14 given the increased space toward the cable end of the transducer. In one such representative example, items 14 would consist of thin films of flexible copper, perhaps in the form of a flexible circuit, extending away from the edges of the piezoelement array 12 and possibly emanating from within an interface such as the interface between backer 11 and array 12, array 12 and layer 13 or layer 13 and lens 3 wherein it also serves an aforementioned electrode function. In this example, the primary purpose of member (or flex circuit) 14 is electrical interconnection as necessary in the interfaces between at least certain of the laminated layers. Items 15 would typically consist of aluminum or copper plates, perhaps between 0.010-0.080 inches thick, which are bonded or thermally coupled intimately to the inner surfaces of case 2. The joint between members 14 and 15 must be thermally conductive. If member 14 is an electrical flex circuit used for interconnection, then care would be taken to provide only a thermal joint and not an electrical joint so as not to short out the flex traces which need to be routed (not shown) backwards to interconnects 10.
As the sonographer or user images with transducer probe 1, the system console 9 transmits a series of electrical pulses through the connectors 7,6 and cable 4 to the acoustic array of piezoelements 12. The electroacoustic piezoelements 12 convert the electrical pulses to acoustic output energy emanating from the rubber lens 3 into the patient. During the ultrasound reception portion of the acoustic beamforming, the piezoelement senses in a passive mode the electrical disturbance produced by acoustic energy bounced off of internal patient tissue and reflected back into the transducer 1. It is primarily the transmit portion of imaging when heat is produced by the piezoelements. This is because the electroacoustic energy conversion process is less than 100% efficient. Thus the piezoelements 12 act as unintended heaters. Secondly, as ultrasound energy is produced by the piezoelements 12, it is somewhat absorbed by layers 13 and lens 3, such layers usually not being totally lossless. The unavoidable nonzero portion of acoustic energy which is directed away from the patient into the backer 11 also serves to generate heat in backer 11. Thus, we have heat being directly generated in the piezoelements 12 and indirectly generated in backing material 11, matching layer(s) 13 and lens 3.
A thermal member 14, if comprised of a flexible circuit being formed in part of a thin metal such as copper, offers modest thermal conduction of heat generated by piezoelements 12 laterally in the X direction to the edges of the device and then downward to some more significant thermal sink, such as 15. The purpose of member 15 is to render isothermal the inner surface of the case 2 so that heat may be encouraged to flow across the case wall at all locations. The thermal purpose of member 14 is to get the heat away from the piezoelements 12 and redirected so that it can be flowed into said isothermalization member 15. Using the combination of thermal elements 14 and 15 it has been possible to passively spread the heat out isothermally to most of the interior case 2 surfaces. It should be understood that case 2, being fabricated of a polymer, will typically conduct heat poorly. It is therefore critical to get the heat spread out over most or all of the interior surface of case 2 so that although the thermal resistance across the thickness of the case wall 2 is high, there is considerable surface area to compensate for this fact and keep the overall thermal resistance between the elements and the environment as low as possible.
Heat which is generated in matching layer(s) 13 and lens 3 may also be conducted downward toward the piezoelements 12 or to their interfacial electrodes (not shown) which can, in turn, pass heat to the edges of the stack for redirection downward in the -Y direction via member 14 for example. When transducer probe 1 is in contact with a patient's tissue, some heat may pass directly into the patient. In any event, the U.L. limitation on skin or tissue temperature severely limits the temperature of the lens, and heat dissipation toward the patient.
Heat which is generated in backing material 11 may be passed to thermal means such as member 15. Member 15 may be arranged to actually envelope or wrap around backer material 11 in the form of a metallic thermal container or can (not shown) in order to facilitate the passage of heat from backing material 11 into thermal member 15 and out of transducer 1.
Thus, the ability of probe 1 to shed heat to the environment is governed primarily by passive free convection of heat from the probe's external surfaces. There is a rather limited capacity to remove heat by natural convection of air past the external probe surface even in this optimal isothermalized example. In practice, given the limits on the temperature of lens 3 and sonographer gripping comfort, it is not possible to dissipate more than a few watts of thermal energy in this passive prior-art manner. Also, different sonographers typically cover different amounts of the probe surface with their hands as they grip it, and in some cases much of the heat is being transmitted by conduction directly into the sonographer's hand(s). This can produce sonographer discomfort and a poor grip. If the only heat dissipating surface and path available is the external case surface dissipating by convection to the atmosphere or by conduction into the patient and/or the sonographer's hand, then severe power dissipation limits of a few watts will apply, particularly to small probes having small surface areas even if that surface area is isothermalized.
Others have attempted to increase the lateral (X-axis) and/or vertical (Y-axis) thermal conductivity of acoustic backing material 11, piezoelements 12 and acoustic matching layers 13. Although these measures may help keep the face of the acoustic array more isothermal particularly for very large array probes, they do nothing to increase the capacity to remove heat from the probe's external surfaces in an improved manner.
An extension of the passive-cooling approach has included an attempt to conduct or spread some of the heat down the length of the attached cable in order to permit the cable to offer more passive convection surface area. This helps the situation only incrementally because of the user-preferred small diameter cable and the difficulty of providing much of a thermally conductive path in such a small diameter cable without compromising the desired flexibility and compactness of the cable. Such an incremental measure is described in U.S. Pat. No. 5,213,103 "Apparatus for and method of cooling ultrasonic medical transducers by conductive heat transfer" by Martin, et al.
As a specific example a copper braid could be routed from the case 2 interior into at least some limited length of the cable 4 adjacent to device 1. This copper braided thermal means may be connected to a thermal means in the case such as depicted member 14, 15 or 14 and 15 or may also serve as item 15 for example. This tact essentially creates additional dissipative surface area on the cable.
It should be noted that for endocavity transducers (probes inserted internally into the human body) heat is dissipated both by direct conduction to the patient's internal tissues and fluids, as well as by the conduction out the cable and convection from the exposed transducer handle which remains external to the patient's cavity. We must also control the maximum surface temperatures attained by these probes.
The second strategy for cooling transducers, which, to the knowledge of the inventors, has not yet been pursued by any medical ultrasound vendor, is to utilize active cooling rather than passive cooling in order to dissipate heat well beyond that which can be passively convected or conducted from the external transducer surfaces. Active cooling means that one provides a means to actively remove heat from the transducer such as by employing a pumped coolant or other active refrigeration means. Using active cooling one may ensure that one is always able to operate the acoustic transducer up to the allowable acoustic intensity limit while also maintaining acceptable surface temperatures regardless of how small the transducer is or how much surface area it offers for cooling relative to its acoustic intensity.
At least part of the reason active cooling has not yet been used is because of the apparent cost, reliability and the ease-of-use issues associated with it. There is a well-established continued trend in the ultrasound industry toward reliable "solid-state" phased array transducers with no moving parts and with excellent chemical resistance to disinfection procedures, including procedures involving total chemical immersion for extended periods. There is a more recent trend toward minimizing the cost of ownership for all medical implements as well as any need to service or repair them. Both of these trends place very severe constraints on any potential active transducer cooling means for use in the hospital, clinic or doctor's office environment.
Finally, one must keep in mind that imaging transducers are plugged into and unplugged from the ultrasound console's various connector ports in a varying personalized manner, thus any active cooling scheme should preferably continue to allow for the freedom to do this and should not substantially complicate the integrity or ease of this connection. Large numbers of connector plug/unplug cycles should also not degrade the performance of the active cooling means. Any active cooling scheme should involve minimal additional maintenance and should be as transparent to the user as possible.